The present invention relates to an improvement in the hot water process of extracting bitumen from tar sands. This invention particularly relates to treatment of effluent discharge and treatment of water storage retention ponds used in retaining effluent discharge waste streams recovered from hot water extraction of bitumen from tar sands.
Tar sands which are also known as oil sands and bituminous sands are siliceous materials which are impregnated with heavy petroleum. The largest and most important deposits of the sands are the Athabasca sands, found in northern Alberta, Canada. These sands underlay more than 13,000 square miles at a depth of 0 to 2000 feet. Total recoverable reserves after extraction and processing are estimated at more than 300 billion barrels --just equal to the world-wide reserves of conventional oil, 60 percent of which is in the Middle East. By way of contrast, the American Petroleum Institute estimated total United States oil reserves at the end of 1965 at 39.4 billion barrels.
The tar sands are primarily silica, having closely associated therewith an oil film which varies from about 5 percent to 21 percent by weight, with a typical content of 13 weight percent of the sand. The oil is quite viscous--6.degree. to 8.degree. API gravity--and contains typically 4.5 percent sulfur and 38 percent aromatics.
The sands contain, in addition to the oil and sand components, clay and silt in quantities of from 1 to 50 weight percent, more usually 10 to 30 percent. The sands also contain a small amount of water, in quantities of 1 to 10 percent by weight, in the form of a capsule around the sand grains.
Several basic extraction methods have been known for many years for the separation of oil from the sands. In the so called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of neutral salt in water, which salt is capable of acting as an electrolyte. The combined mass is then subjected to a pressure or gravity separation.
In the hot water method, as disclosed in Canadian Pat. No. 841,581 issued May 12, 1971, the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures of 170.degree. to 190.degree.F., and the resulting pulp is then dropped into a turbulent stream of circulating hot water and carried to a separation cell maintained at a temperature of about 185.degree.F. In the separation cell, sand settles to the bottom as tailings and oil rises to the top in the form of a froth. An aqueous middlings layer comprising clay and silt and some oil is formed between these layers. This basic process may be combined with a scavenger step for further treatment of the middlings layer obtained from the primary separation step to recover additional amount of oil therefrom.
The middlings layer either as it is recovered from the primary process or as it is recovered after the scavenger step comprises water, clay and oil. The oil content is, of course, higher in middlings which have not undergone secondary scavenger steps.
As disclosed herein the term effluent discharge is used to describe waste water streams associated with the hot water method for extracting bitumen from tar sand which streams contain at least 5 weight percent sand and are not the primary products of the hot water process. The effluent discharge is removed from the process plant as a slurry of about 35 to 75, typically 45 percent, solids by weight. Included in the slurry is sand, silt, clay and small quantities of bitumen. In this specification, sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh but is larger than 2 microns. Clay is material smaller than 2 microns including some siliceous material of that size. Included in the slurry is sand, clay and small quantities of bitumen ranging from about 0.5 to 2.0 weight percent of the total discharge. The discharge stream on an average contains 35 to 45 weight percent sand.
Because the effluent contains oil emulsions, finely dispersed clay with poor settling characteristics and other contaminants, water pollution considerations prohibit discarding the effluent into rivers, lakes or other natural bodies of water. The disposal of the effluent discharge has therefore presented a problem. Currently, effluent discharge is stored in evaporation ponds which involve large space requirements and the construction of expensive enclosure dikes. A portion of the water in the effluent discharge is recycled back into the hot water extraction process as an economic measure to conserve both heat and water. However, experience has shown that the dispersed silt and clay content of the recycled water can reduce primary froth yield by increasing the viscosity of the middlings layer and retarding the upward settling of oil flecks. When this occurs, the smaller oil flecks and those that are more heavily laden with mineral matter stay suspended in the water of the separation cell and are removed from the cell with the middlings layer.
Effluent discharge from the hot water process for extracting bitumen from tar sands contains a substantial amount of mineral matter some of which is colloidally dispersed in the effluent discharge and therefore does not settle very readily when stored in the retention pond. The lower layer of the retention pond can contain up to 50 percent dispersed mineral matter made up substantially of clay and silt and can contain up to 5 percent bitumen. This part of the pond water is normally referred to as sludge. This part of the pond is generally not suitable for recycling to the hot water extraction process for the reason that its addition into the separation cell or the scavenger cell at the normal inlet means would raise the mineral content of the middlings of the cell to the extent that recovery of bitumen would be substantially reduced. Generally, the settling which does take place in the pond provides a body of water in which the concentration of mineral matter increases substantially from the surface of the pond to the bottom thereof. One such pond now in commercial use containing effluent discharge and having a surface area of about 1000 acres and an average depth of 40 feet can be characterized somewhat as follows:
a. From the surface of the pond to a depth of 15 feet, the mineral concentration which is primarily clay is found to be about 0.5 to 5.0 weight percent. This pond water can normally be recycled to a hot water extraction process without interfering with the extraction of bitumen from tar sands.
b. The layer of water in the pond between 15 and 25 feet from the surface contains between 6 and 15 percent mineral matter. This water if recycled in any appreciable extent in lieu of fresh water to the separation cell feed with fresh tar sands would increase the mineral content of the middlings portion of the cell to the point that little bitumen would be recovered.
c. Finally, the section of the pond between 25 feet and the bottom of the pond contains 16 to 50 percent mineral matter and is normally referred to as sludge.
Any pond associated with hot water extraction of bitumen from tar sands and containing a sludge layer containing minerals as defined above is suitable for use in the method of the present invention.